You can capture just the active window (useful if you have a multiple monitor setup) by pressing Atl+PrintScreen. It will stay on your clipboard as the previous poster mentioned. Just paste in where you want it.

If you're going to email the screenshot, most major email clients (Thunderbird, TrulyMail, etc.) allow you to simply 'paste' an image from your clipboard into the body of a message. In this case, there is no need to use another application to save the image before sending it.